A wheel axle suspension of a trailer or a lorry has a trailing arm on each side of the vehicle which extends substantially in the longitudinal direction of the vehicle. The axle body of the wheel axle is attached to the trailing arms. Each trailing arm is hingedly connected at the front, viewed in the direction of travel of the vehicle, to a bearing bracket which is arranged on the vehicle chassis. Generally, a pneumatic spring is arranged between the trailing arm and the chassis of such a wheel axle suspension. A flexible trailing arm commonly comprises a spring portion, an eyelet at the front end of the spring portion for hingedly mounting the trailing arm to the vehicle, and a mounting portion at the rear end of the spring portion where the wheel axle body can be mounted to the trailing arm.
In the field of manufacturing flexible trailing arms it is common to manufacture the trailing arms by rolling a heated steel blank into a desired shape of the trailing arm. In general this is the shape of a leaf spring. The trailing arm is commonly shaped in several rolling steps, and possibly bending and cutting steps. The forming includes a step where a hinge eyelet is bended on the front end of the rolled flexible trailing arm. These forming steps take place consecutively and the intermediary product is conveyed between consecutive rolling and bending devices. Before each of the rolling and bending steps the trailing arm may be reheated, at least partly. After the product is entirely shaped it is allowed to cool in air to room temperature. The shaped steel arm is given the desired mechanical properties, in particular the desired tensional strength, by hardening the steel which includes reheating the shaped arm such that the steel becomes an austenite structure and subsequently quenching it in an oil bath of about 80° C., whereby the steel obtains a martensite structure with a sufficient tensile strength for the flexible trailing arm to perform its function properly. The quenching in oil is hazardous in view of fire risks. Furthermore the oil is a danger for the environment. The resulting martensite structure after quenching is very hard and brittle, which is undesired for the flexible trailing arm, which in use is subjected to dynamic loads. Therefore it is necessary to temper the steel after hardening it by reheating it again to a moderately high temperature (e.g. at 180-220° C.) and keep it at that temperature for one or two hours.